1. Field of the Invention
The present invention relates generally to a microwave oven and method of controlling the same, and more particularly, to a microwave oven and a method of controlling the microwave oven to recognize a power supply frequency correctly at the time of power supply by eliminating the influence of noise.
2. Description of the Related Art
In general, a microwave oven is an apparatus for heating and cooking food using microwaves, which includes various drive devices, such as a high voltage transformer, a magnetron and the like, and is supplied with power from the outside to operate the drive devices. The microwave oven generates external interrupts corresponding to a power supply frequency and calculates cooking time using these interrupts. Since the power supply frequency is different depending on the installation environment of the microwave oven, for example, 50 Hz or 60 Hz, the microwave oven must correctly recognize the power supply frequency supplied.
When the power code of the microwave oven is applied to a power supply outlet, the power supply becomes unstable due to the defective application of the power code to the outlet, signal delay or the like. Thus, it is difficult to correctly measure the power supply frequency.
In consideration of this problem, in the conventional microwave oven, a power supply frequency is measured when a certain time elapses after the power has been supplied. As shown in FIG. 1, when external interrupts corresponding to a certain frequency are input, the measurement of the power supply frequency is deferred during a first period for which a power supply signal of twenty cycles is input. Thus, after the first period has elapsed, the power supply frequency is measured during the next 20 cycles of the second period. Since the power supply signal includes delay signals E1 and E2, the power supply frequency can be recognized incorrectly due to the delay signals E1 and E2. Hence, the power supply frequency needs to be measured a plurality of times, for example, twenty times.
FIG. 2 is a flowchart illustrating a conventional method of controlling the microwave oven. When power is supplied to the microwave oven by applying the power code to the outlet, an inner timer is operated and a timer count B is increased at operations 70 and 72, respectively. Thereafter, it is determined whether external interrupts are generated at operation 74. If the external interrupts are generated, the first number of times A the external interrupts are generated is increased cumulatively at operation 76.
Thereafter, it is determined if the accumulated first number of times A exceeds twenty at operation 78. If the first number of times A is less than twenty, the timer count B is cleared at operation 79 and proceeds to operation 70 to accumulate the first number of times A continuously. If the first number of times A exceeds twenty, that is, if the first period for which the power supply frequency is not measured elapses, it is determined whether the timer count B reaches a set value, for example, fifteen, that discriminates between a first power supply frequency of 50 Hz and a second power supply frequency of 60 Hz at operation 80. If the timer count B reaches fifteen, a second number of times C is accumulatively increased at operation 82. Thereafter, it is determined if the first number of times A equals forty at operation 84. If the timer count B is less than fifteen, the process proceeds to operation 84.
As the result of the determination at operation 84, if the first number of times A is less than forty, the process proceeds to operation 79 where the timer count B is cleared, and then proceeds to operation 70. As the result of the determination at operation 84, if the first number of times A equals forty, that is, if the second period elapses, it is determined if the second number of times C is equal to or larger than ten at operation 86. If the second number of times C is equal to or larger than ten, the power supply frequency is set to a first frequency of 50 Hz at operation 88. In contrast, if the second number of times C is smaller than ten, the power supply frequency is set to a second frequency of 60 Hz at operation 90.
However, the conventional microwave oven is problematic in that the conventional microwave oven is affected significantly by high frequency noise at the time power is supplied. When an external interrupt includes high frequency noise as shown in FIG. 3, five or six extraneous external interrupts are generated even though only one external interrupt is actually generated. For this reason, the first period for which the measurement of the power supply frequency is deferred is shortened, and the power supply frequency is measured in a state of unstable power. For example, in the case where the power supply frequency of 50 Hz, including high frequency noise, is measured, the timer count B is cleared before the timer count B reaches fifteen, and the second number of times C is less than ten, so the power supply frequency is determined to be 60 Hz. Accordingly, the power supply frequency may be recognized incorrectly.
In consideration of the above-described problems, a method employing a low pass filter may be used in the microwave oven to eliminate high frequency noise. However, this conventional method is disadvantageous because the manufacturing cost of a microwave oven is increased by adding an expensive part thereto.